The exemplary embodiments described herein relate generally to semiconductor devices and methods for the fabrication thereof and, more specifically, to the structures and fabrication methods of lateral bipolar junction transistors having multiple base lengths.
A bipolar junction transistor (BJT) is a semiconductor device comprising three sections of semiconductor material arranged to alternate between P-type and N-type, the sections of semiconductor material forming a base, an emitter, and a collector, thus resulting in a three-region device having an emitter region, a base region, and a collector region having two P-N junctions with one P-N junction being between the emitter and the base and the other P-N junction being between the collector and the base. Each bipolar junction transistor is thus classified as either PNP or NPN according to the arrangement of the P-type material and N-type material. An NPN BJT has an N-type emitter, a P-type base, and an N-type collector, and a PNP BJT has a P-type emitter, an N-type base, and a P-type collector. The function of a BJT is to amplify current, i.e. the collector current is larger than the base current.
In a lateral BJT, the base is located between the emitter and the collector, with the emitter/base junction and the collector/base junction being formed between laterally arranged components. The characteristics of a lateral BJT depend on the length of the base, which is determined by the spacing between the emitter and the collector regions. For digital circuits, a small base length is desirable in order to increase speed. For radio frequency (RF) and analog applications, high speed (high unity current gain frequency (fT) and high unity power gain frequency (fmax)) is desirable for some applications, and large Early voltage (VA) is desirable for other applications. Early voltage is derived from a relationship between collector current as a function of collector/emitter voltage and is based on the Early effect, which is the variation in the length of the base in the BJT due to a variation in the applied base-to-collector voltage. However, in the BJT there is a tradeoff between speed and Early voltage.